This invention relates to a sensing device incorporating a single crystal semiconductor wafer and more particularly to a sensing device having a low cost, reduced blooming target.
Sensing devices such as silicon vidicons and silicon intensifier tubes employ sensing elements or targets comprising single crystal silicon wafers. The operation of such sensing elements in these devices is well known in the art. The phenomenon of blooming, common in silicon targets, is described in detail in "High Light-Level Blooming in the Silicon Vidicon" by E. C. Douglas, in "IEEE Trans. on Electron Devices," Vol. ED-22, pages 224-234, May 1975 and incorporated by reference herein. Attempts to control blooming in such devices are described in "Theory, Design, and Performance of Low-Blooming Silicon Diode Array Imaging Targets" by B. M. Singer and J. Kostelec in "IEEE Trans. on Electron Devices," Volume ED-21, pages 84-89, January 1974. Blooming Control in the Singer reference is achieved by forming an N+ potential barrier by ion implantation of phosphorous on the light input side of the silicon target. Blooming control as taught by Singer et al. exhibits uncontrollable variations and instabilities of dark current and blooming control performance.
Recently, improvements have been made in reducing and stabilizing the blooming effects of these targets as described in a copending patent application, Ser. No. 838,713, filed Oct. 3, 1977 by Savoye et al., entitled, "Reduced Blooming Devices", now U.S. Pat. No. 4,232,245 issued on Nov. 4, 1980, assigned to the same assignee as the present application, and incorporated by reference herein. As described in the aforementioned application, the blooming characteristics of the target are reduced by the ion implantation of an N+ potential barrier spaced less than about 1500 A from the light input surface. The blooming characteristics are stabilized by depositing a passivating layer of boron-containing silica glass onto the input signal sensing surface of the silicon target.
The quantum efficiency of the aforementioned target has been enhanced by coating the passivating layer with a material which, in combination with the passivating layer, forms an antireflective region having an optical thickness substantially equal to an odd multiple of a quarter wavelength of the light incident on the device. Such a structure is described in a copending patent application, Ser. No. 037,832, filed May 10, 1979 by W. M. Kramer, entitled, "Reduced Blooming Device Having Enhanced Quantum Efficiency", now U.S. Pat. No. 4,228,446 issued on Oct. 14, 1980 assigned to the same assignee as the present application and incorporated by reference herein.
The silicon target structures described in both the Savoye et al. patent application and the Kramer patent application provide a high performance, reduced blooming target; however the cost of such a target is significant. Targets having ion implanted potential barriers such as those produced by the methods described in the Sayove et al. application have relatively low manufacturing yields because of cosmetic defects, such as white spots, which are caused by inadequate surface cleaning techniques and because of the requirement that the ion implanted potential barrier have a doping profile with the peak of the dopant concentration being located less than about 1500 A from the input surface of the target.
In order to produce a low cost, reduced blooming target it is necessary to either improve the manufacturing yield of targets produced by the above described process, or to simplify the target processing while maintaining adequate blooming control. In certain applications high performance reduced blooming targets such as those described in Savoye et al. and the Kramer patents are not required and targets having good or adequate blooming control are desirable.
Such a target is described in British Pat. No. 1,337,206 to Ohkubo et al. published Nov. 14, 1973, and entitled, "Silicon Target for Image Pick-Up Tube". The aforementioned patent describes a silicon target structure without an ion implanted potential barrier, but having a plurality of periodic regions on the light input side of the target for recombining minority carriers generated by the projection of light onto the silicon target. These recombination regions reduce blooming by causing laterally diffusing minority carriers to recombine before they can discharge adjacent diodes on the scanned side of the target. The recombination regions are disclosed to be produced by ion implantation and cover approximately 20% of the light incident surface. An N+ region is subsequently formed on the light input side of the target by diffusion. The patentees claim that a reduction in sensitivity occurs if the recombination regions exceed about 20% of the total area of the light incident surface. The reduction in sensitivity is allegedly caused by the presence of the recombination regions.
A problem caused by the presence of the periodic recombination regions disclosed by the patentees is that under no light, or low light illumination the periodic pattern of the recombination regions can appear in the dark current output of the tube and form an objectionable pattern when displayed on a monitor. Such a pattern is objectionable and must be eliminated without sacrificing blooming control or further reducing target sensitivity.
Another method for reducing blooming is described in U.S. Pat. No. 3,895,430 to Wilson et al. issued on July 22, 1975, and entitled, "Method for Reducing Blooming in Semiconductor Array Targets." The Wilson et al. patent describes a method for forming recombination sites between the diodes by irradiating the diode side of the target with electrons, ions or photons and using the diode caps as a mask. The recombination sites located between adjacent diodes exist at the interface between the substrate and the insulation layer and therefore are less effective in controlling blooming than recombination sites located within the wafer.